It is known that compounds containing olefinic double bonds can be reacted with carbon monoxide and hydrogen to form aldehydes (oxo process). The process is not restricted to the use of olefinic hydrocarbons but also extends to starting materials which have not only the double bond but also functional groups, predominantly groups which remains unaltered under the reaction conditions.
The classical oxo process is carried out using cobalt as catalyst. Its effectiveness is based on the formation of cobalt carbonyl compounds by action of hydrogen and carbon monoxide on metallic cobalt or cobalt compounds at pressures above 20 MPa and temperatures of about 120° C. and above.
During the further development of the oxo process, cobalt has been increasingly replaced by rhodium as catalyst metal. Rhodium is used as a complex preferably containing phosphines as ligands in addition to carbon monoxide. Rhodium as metal allows the process to be carried out at lower pressures, and in addition higher yields are achieved and the unbranched products which are more valuable for further processing are preferentially formed when straight-chain terminal olefins are used as starting materials.
Industrially, the hydroformylation of olefinically unsaturated compounds is carried out under the catalytic action of rhodium carbonyl complexes having tertiary organic phosphines or phosphites as ligands. In one process variant, the reaction is carried out in a homogeneous organic phase, i.e. olefinically unsaturated compound used, catalyst and the reaction products of the hydroformylation reaction are present together in solution. The reaction products are usually separated off from the mixture by distillation, more rarely by other methods such as extraction. The hydroformylation process carried out in the homogeneous phase can be configured as a gas recycle process as described in U.S. Pat. No. 4,247,486 A1 or in the form of a liquid recycle process as described in U.S. Pat. No. 4,148,830 A1.
High-boiling aldehyde condensation products, which comprise a complex mixture of oxygen-containing compounds which are formed from the initially formed aldehydes by means of condensation reactions such as the aldol reaction or by the Tishchenko reaction and have different degrees of condensation, are formed as by-products in the hydroformylation reaction.
The formation of, for example, dimers, trimers or tetramers as high-boiling aldehyde condensation products in the hydroformylation reaction carried out homogeneously is described, for example, in U.S. Pat. No. 4,148,830 A1. These high-boiling aldehyde condensation products are suitable as solvent for the homogeneously dissolved rhodium catalyst. In addition, they stabilize the rhodium catalyst both in the hydroformylation reaction and in the subsequent work-up steps for the liquid hydroformylation output. However, to prevent their concentration in the liquid phase from increasing too greatly, part always has to be removed from the hydroformylation process. In steady-state operation of the hydroformylation plant, the high-boiling aldehyde condensation products are discharged in the same amount in which they are formed.
In the liquid recycle process as described in U.S. Pat. No. 4,148,830 A1, the liquid output from the hydroformylation reaction is firstly depressurized into a depressurization vessel in which separation into a gas phase and a liquid phase occurs. The liquid phase contains essentially the catalyst, high-boiling aldehyde condensation products, solvent, the desired aldehyde and small amounts of unreacted olefinically unsaturated compound. The gas phase contains excess synthesis gas, inerts and hydrogenated products, small amounts of the aldehyde formed and also unreacted olefinically unsaturated compounds. After a proportion of inerts and hydrogenation products have been separated off and removed, the gas phase is compressed and recirculated to the hydroformylation reactor. The liquid catalyst-containing stream is fed to a separation apparatus, preferably a distillation apparatus, from which the desired aldehydes and small amounts of dissolved synthesis gas and olefinically unsaturated compound are taken off as overhead product. The mixture is then separated into the pure n- and iso-aldehydes in a further purification step. The rhodium-containing stream obtained, which additionally contains the high-boiling aldehyde condensation products as solvent, is recirculated to the hydroformylation reactor.
WO 01/58844 A2 relates to the work-up of a liquid output from a hydroformylation reaction which is carried out continuously. In the known process, a two-stage depressurization is carried out, with depressurization to a pressure which is from 0.2 to 2 MPa below the reaction pressure being carried out in the first stage. The resulting liquid phase is depressurized further in a second, lower-pressure stage to form a gas phase which contains essentially the major part of the desired hydroformylation product. The liquid phase obtained, which contains high-boiling by-products and the homogeneously dissolved hydroformylation catalyst, is recirculated either directly or after work-up by distillation to the hydroformylation reactor.
It is known that decomposition and/or deactivation of the rhodium catalyst can occur in the work-up by distillation of the reactor output from the rhodium-catalyzed hydroformylation reaction carried out homogeneously. Such decomposition and/or deactivation can result, firstly, in precipitation of insoluble rhodium compounds or rhodium metal which deposit as deposits or coatings in the apparatuses and can no longer be recirculated in the hydroformylation process carried out homogeneously. Such amounts of rhodium are, firstly, no longer available to the ongoing process and can only be recovered from the apparatuses by means of specific cleaning measures during a shutdown of the plant. Furthermore, rhodium-containing solids formed by decomposition and deactivation can be present in suspended and thus pumpable form in the distillation residue and be able to be recirculated to the hydroformylation reactor, but such suspended rhodium-containing solids frequently have only a low hydroformylation activity. This can likewise apply to decomposition and deactivation products which are present in dissolved form in the distillation residue. To maintain a prescribed space-time yield of aldehyde, it is therefore necessary to introduce more rhodium in the form of fresh rhodium catalyst than the amount of rhodium taken off with the high-boiling aldehyde condensation products formed. The difference between the amount of fresh rhodium added and the amount of deactivated rhodium discharged remains in the plant and initially counts as rhodium consumption.
It is therefore an object of the invention to provide a process for working up a liquid output from a hydroformylation reaction, in which the decomposition and/or deactivation of the homogeneously dissolved rhodium complex catalyst can be suppressed effectively.